1. Field of the Invention
The present invention relates to a cross current compensation control system for a power system which compensates a cross current during the parallel operation of a power transmission line, a power transformer, a device such as a distribution and equipment in the power system. For convenience of description, the operation of the distribution system will be mainly described.
2. Description of the Related Art
A conventional parallel operation of the distribution system will be described with reference to the accompanying drawings. FIG. 15 is a circuit diagram showing a conventional distribution parallel operation of the distribution system.
Referring to FIG. 15, reference numeral 1 denotes a power supply; 2 is a primary side circuit breaker of a power transformer (Tr); 3 is a primary side winding of the power transformer; 4 is a secondary side winding of the power transformer; 5 is a secondary side circuit breaker of the power transformer; and 6 is a bus bar.
Also, in the figure, reference numeral 10 denotes a distribution; 10-1 is a circuit breaker of the distribution 10; 11-1, 11-2, 11-3, 11-4 and 11-5 are sections in the distribution 10; and 10-2, 10-3, 10-4 and 10-5 are section switches or circuit breakers (hereinafter referred to as “switches”) of the distribution 10.
Also, in the figure, reference numeral 20 denotes a distribution; 20-1 is a circuit breaker of the distribution 20; 21-1, 21-2, 21-3, 21-4 and 21-5 are sections in the distribution 20; and 20-2, 20-3, 20-4 and 20-5 are section switches or circuit breakers (hereinafter referred to as “switches”) of the distribution 20.
In addition, in the figure, reference numeral 12 denotes a CT (current transformer) of the distribution 10; 13 is a device that monitors and protects the distribution 10 (hereinafter referred to as “protection relay” for convenience of description); 22 is a CT (current transformer) of the distribution 20; 23 a device that monitors and protects the distribution 20 (hereinafter referred to as “protection relay” for convenience of description); and 30 is a distribution connection switch or circuit breaker (hereinafter referred to as “switch”), between distribution 10 and 20.
In FIG. 15, the CT 12 of the distribution 10 and the CT 22 of the distribution 20 derive currents in the respective phases by using three CTs in a direct ground system of the distribution system, and an over-current relay (one of the protection relays) is located at their secondary sides, respectively, to protect against the over-load and short-circuiting fault of the distributions 10 and 20. In addition, a ground over-current relay (one of the protection relays) is disposed in a residual circuit at the secondary side of each of the CTs 12 and 22, respectively, to protect against the ground fault. Also, a CT and a protection relay are disposed in each of the section switches 10-2, 10-3, . . . , 20-2, 20-3, . . . and 30, as in the sending ends of the distributions 10 and 20, to monitor and protect the respective sections 11-2, 11-3, . . . and 21-2, 21-3, . . . .
Also, in FIG. 15, the CT 12 of the distribution 10 and the CT 22 of the distribution 20 derive currents in the respective phases by using two CTs in a high resistance ground system (including a GPT ground system) of the distribution system, and an over-current relay is located at their secondary sides, respectively, to protect against the over-load and short-circuiting fault of the distributions 10 and 20. Also, the CT 12 and the CT 22 derive a zero-phase current and a zero-phase voltage by using a ZCT and a GPT, and generally connect a ground direction relay to conduct a ground protection. Also, a CT and a protection relay which are similar to those at the sending ends of the distributions 10 and 20 are disposed in each of the section switches 10-2, 10-3, . . . , 20-2, 20-3, . . . and 30, to monitor and protect the respective sections 11-2 11-3, . . . and 21-2, 21-3, . . . .
Subsequently, the operation of the conventional distribution system will be described with reference to the accompanying drawings.
It has been known that a cross current 50 of a zero-phase flows when the switch 30 is closed by the operation of the distribution in FIG. 15. This is because the cross current 50 of the zero-phase occurs because impedances in the respective phases in a closed loop have slight differences even if supply voltages are identical.
The occurrence of the cross current will be described now. FIG. 16 shows a principle diagram of the generation of across current. In the figure, references Ea, Eb and Ec denote supply voltages; Za1, Zb1 and Zc1 are impedances at the distribution 10 side; and Za2, Zb2 and Zc2 are impedances at the distribution 20 side.
In this example, an actual distribution system is that when the switch 30 is closed, the above impedances are not completely identical with each other strictly, but very slightly in an unbalanced state in the respective phases. The unbalance is caused by, for example, a slight difference of the lengths of electric wires in the respective phases and a light difference of the contact resistances at nodes of the respective electric wires. Also, the unbalance is caused by a difference of a voltage drop in each of the phases due to the load unbalance of each of the phases, etc. There has been known a fact that an unbalanced voltage develops within a loop even if the supply voltage is of a synthetic three-phase voltage.
That is, in FIG. 16, the voltages in the respective phases when the switch 30 is opened are represented by a table of FIG. 17. In the table, the phase voltages at the distribution 10 side are Ea1, Eb1 and Ec1, and the phase voltage at the distribution 20 side are Ea2, Eb2 and Ec2, and differential voltages of the respective phases are represented by ΔEa, ΔEb and ΔEc. It is general that those differential voltages ΔEa, ΔEb and ΔEc cannot become the synthetic three-phase voltages due to a difference of the impedances of the respective phases and a difference of the load currents of the respective phases, as described above.
Subsequently, the magnitude of the cross current will be described. A positive-phase voltage, a negative-phase voltage and a zero-phase voltage called in a method of symmetric coordinates exist in an asymmetric three-phase voltage, and it is apparent that the magnitude of the cross current is determined by those voltages and a positive-phase impedance, a negative-phase impedance and a zero-phase impedance of the closed circuit. Although being dependent on the system, in this case, a fact that the magnitude of the cross current of the zero phase becomes about 1 to 10 A has been observed in the actual system.
The primary rating of the CT as generally used is 400 A or 600 A. In this example, the positive-phase and negative-phase currents are relatively small in comparison with the detection level of the over-load and the short-circuiting protection relay (about 5 A to 6 A of the CT secondary rating or more), and the zero-phase current becomes a value close to the detection level (about 0.1 A to 0.5 A) of the ground protection relay of the direct ground system. It is readily presumable that the current exceeds the detection level of the ground protection relay (about 0.2 to 0.5 A at the ZCT primary side).
The parallel operation of another conventional distribution system will be described with reference to the accompanying drawings. FIG. 18 is a circuit diagram showing a transformer in the same power supply in the conventional distribution system and the parallel operation of the distribution system. Also, FIG. 19 is a circuit diagram showing a transformer in the same power supply in the conventional distribution system and the parallel operation through the distribution system.
FIGS. 18 and 19 are diagrams showing the parallel operation of the power transformer and the parallel operation of the distribution. In the figures, reference 2-1 denotes a primary side circuit breaker; 3-1 is a primary side winding of the power transformer; 4-1 is a secondary side winding of the power transformer; 5-1 is a secondary side circuit breaker; 6-1 is a bus bar; and 7 is a bus bar connection circuit breaker. Other symbols are identical with those in FIG. 15.
The cross current will be described. In FIG. 18, when the switch 30 of the distributions 10 and 20 is closed, the cross current 50 occurs as described above. When the bus bar connection circuit breaker 7 is closed, the cross current 50-1 occurs. The occurrence causes of the cross current 50-1 are generally the impedance difference described above with reference to FIG. 15 as well as a voltage adjustment tap attached to the power transformer. The difference of the voltage adjustment tap position, and the impedance difference of the power transformer greatly operate as a source of generating the cross current. In this case, there has been well known a fact that all of the cross currents of the positive phase, the negative phase and the zero phase may lead to a problem.
Subsequently, the magnitude of the cross current will be described. In FIG. 18, the cross current 50 in the case where the bus bar connection circuit breaker 7 and the switch 30 are closed is identical with that in case of FIG. 15, and therefore its description will be omitted. A crosscurrent 50-1 will be described.
The short-circuiting % impedance (Z) of the power transformer is about 5 to 10%, and a voltage of one tap of the voltage adjustment tap is about 1 to 2%. In this example, assuming that % Z is 7.5% in each of the power transformer in both of two power transformers, a one-tap voltage is 1.25% and the shift is two taps, the subsequent values are obtained through rough calculation.
Differential voltage ΔV=1.25%*2=2.5%
Closed loop Z=7.5%*2=15%
Cross current I=ΔV/Z=16.7%
That is, in the power transformer where the secondary rating is 10 MVA, 6.6 kV and 875 A, the magnitude of the cross current in this case becomes 875 A*0.167=146 A and thus becomes a very large value.
Subsequently, a case in which the bus bar connection circuit breaker 7 is opened and the switch 30 is closed in FIG. 19 will be described. In this case, the cross current becomes in a state where the cross current described in FIG. 15 and the cross current in the above parallel power transformer are superimposed on each other. The cross current of the zero-phase of the high-voltage distribution becomes much-larger as compared with that in case of FIG. 15 and may reach several tens of A to about 100 A depending on the circumstances.
The parallel operation of still another conventional distribution system will be described with reference to the accompanying drawings. FIG. 20 is a circuit diagram showing a transformer in a different power supply system of the conventional distribution system and the parallel operation of the distribution system. Also, FIG. 21 is a circuit diagram for explanation of a cross current that occurs during the parallel operation in the different power supply in the conventional distribution system.
FIG. 20 shows a case of the parallel operation of a two power supply system. In FIG. 20, reference 1-1 denotes another power supply. The same references denote identical parts in FIG. 18.
The cross current will be described. Referring to FIG. 20, the cross current in the case where the bus bar connection circuit breaker 7 is opened and the switch 30 is closed will be described. The cross current 50 that occurs in FIG. 20 is added with the magnitude of the voltages of two power supplies and the voltage phase in addition to the case of FIG. 18. In general, the parallel of the two power supplies are permitted only when the power supply 1 and the power supply 1-1 are identical with each other in its upstream system, and the system operation is made. It is general that the parallel in the completely different systems is not implemented.
Subsequently, the magnitude of the cross current will be described. FIG. 21 is a diagram showing a power supply and an impedance corresponding to FIG. 20. As described above, the voltages, the impedances and the voltage phases in two power supplies determine the magnitude of the cross current. The voltages Ea-1, Eb-1, Ec-1 and Ea-2, Eb-2, Ec-2 shown in the figure are applied from the same bus bar of the upstream system or a higher upstream system, and in any case, a difference in the magnitude and phase exists between those voltages. The magnitude of the cross current is determined in accordance with the voltage, the phase difference and the magnitude of the impedance within the closed loop.
The parallel operation of the conventional distribution system shown in FIG. 15 suffers from such a problem in that the existing ground protection relay is adversely affected by the cross current, in particular, the zero-phase current. That is, a purpose of the protection relay may be lost such that the ground fault current to be operated is offset into malfunction or malfunction is made at a time where operation should not be conducted, in accordance with the direction of the cross current of the zero-phase. Also, an ohm loss may occur due to the positive-phase and negative-phase cross currents or an excessive power (an active component, a reactive component) is measured in the measured value.
Also, in the parallel operation of the conventional distribution system shown in FIG. 18, there is a case in which the cross currents may exceed the rated current of the power transformer because the cross currents of the positive phase and the negative phase between the power transformers become very large values in addition to the case shown in FIG. 15, which influences the various protection relays and also influences the measured value. In particular, in the case of conducting the parallel operation only at the distribution side, it can be readily presumed that the function of section monitoring terminals located in the distribution and the sections is remarkably impeded.
In addition, the parallel operation of the conventional distribution system shown in FIG. 20 suffers from the same problem as that in FIG. 18 and has the possibility that a severe problem occurs.
The parallel operation technique of the conventional power system suffers from the above problems and resulting in that:
1) the existing protection relay cannot achieve a desired object, in particular the ground relay suffers from a problem;
2) a useless value is contained in the existing measured value;
3) a useless loss occurs in the power system;
4) an overload is induced; and
5) a power device is damaged.